Quote:
Originally Posted by kach22i
Is there a pressure wave of some kind hovering over an automobile because of the ground plane interaction?
If so, then this means we need blunter noses to build up attachment and longer bodies (compared to free-air) to maintain it?
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It was Paul Jaray who discovered that a low drag body in free flow would exhibit a two-fold drag increase when near the ground plane.
A vehicles drag,as far as the atmosphere is concerned,is represented by the vehicle and it's mirror image below the ground.
So your car is only half as long,or twice as high as you observe it,aerodynamically.
Jaray determined that the lowest drag form is half of a streamline body of revolution and its complementing mirror image below the ground.
The 'Template' is derived from a 2.5:1 streamline body of Cd 0.04 which creates a body in ground-effect which is five times as long as it is high of Cd 0.08.
At this 'length' the rear contour just respects Mair's maximum boat tail angle of 22-degrees.
If you build the car shorter,the angles are too steep,you get separation and higher drag.
If you build the car longer your pressure drag isn't any lower and the increased wetted area causes higher overall drag due to the increased friction.
The 2.5:1 ratio is the 'sweet-spot',balancing lowest pressure drag and lowest surface friction.
That's all there is to it.
All pressures around the car will be a function of local flow velocity and vary from location to location.
The bulbous nose of the 'Template' provides gentle uniform flow accelerations,minimum pressure spikes,and vectoring to energetic longitudinal flow to the point of maximum body cross section,after which,the gentle converging aft-body provides the gentle pressure rise necessary to protect the fragile turbulent boundary layer which is operating in an adverse pressure regime,with nothing there to hold it against the body except that gentle contour.